Loading system



July 8, 1930.

s. P. MEAD ET AL 1,769,959

LOADING SYSTEM Filed Aug. 18 1926 K MOR/VE Y Patented July s, 193e @TES unire 'e Aram opinies SALLIE alunni), or New Yann, AND Noni/IAN annuncia, (JF-BROOKLYN, NEW YORK,

AssIGNoas To AMnnrcAN TELEPHONE AND 'rnnneimrn COMPANY, A CORPORA- V'rIoN on NEW YORK LOADING SYSTEM Application led August 18, 1926. Y Serial No. 130,074.

This invention relates to compound loading systems and'particularly to` a system of that typer having its elements so proportioned as to provide a plurality of signaling'channels, each representing a band of frequencies separated from each other.

The copending` application of N. R. French, Serial No. 68,692, iiled November l2, 1925, which issued as Patent No. y1,741,926 on Dec. 3l, 1929 discloses a compound system of loading characterized bythe fact that cach loading coil has an inductance value different from that of the coils connected with the same circuit at adjacent loading points thereon, the alternate coils having the saine inductance value. By means of-,such a system of loading it is possible to give to a loaded cable circuit the same impedance and thev same critical frequency as possessed by a loaded open-wire line connected with the cable circuit. ln such a system no impedance irregularity exists at the unction point between the open wire and the cable, and furthermore, the system permits spacing of the coils so that they may be spliced into the cable at the same loading points at which the loading coils are connected with the trunk circuits which are made up entirely of cable conductors.

lt has been found that by proper proportioning of the various factors involved in system of compound loading thesystem may be made to have two bands'of transmission separated by a band ofsuppression, the vwidth of the latter being controllable.

Our invention resides in a compound system of loading which is so proportioned as to have two distinct bands of transmission whose separation may be varied within limits.

This invention will be clearly understood from the following description when read in connection with the attached drawing, in which Figures l, 2 and 3 illustrate schematically various types of compound'loading, and Fig. el is a graph which serves to show how the bands of transmission and Asuppressi n may be varied.

ln Fig. l, L1 represents a'loading `c'oiljof relatively large inductance and Lg-/a coil of smaller inductance. As indicated in the ligure these coils alternate "throughout the length of the loaded conductor. The varrangement'shown in Fig. l represents the loading upon a cable circuit having uniformly distributed Acapacity throughout. The distance between adjacent loading` coils is likewise the same throughout the length of the cable.

It has been found that in the simple loading systems, such as are common atv the present time, in which coils vof substantially equal inductance arefconnected with thecable circuit at equal distances apart, the resulting loaded circuit has only one transmitting range from Zero to the critical frequency. This critical frequency depends only on the inductance and the spacing of theloading coils. This simple loading also represents a limiting case of compound loading wherethe total inductance of a loading section is concentrated in each alternate loa-ding coil, which result is obtained by reducing to Zero the inductance of the coil between adjacent alternate coils.

The nature of our invention will be apparent if we assume that a small part of the inductance of asimple loading system is taken from the loading coilat each end of the loading section and placed at the center, keeping the total inductance per compound loading section the same. It will be found that,l j j '(1) An additional `transmitting band will be introduced above the original critical frequency.V

(2) The width of this secondary transmittingband and rits position in the frequency spectrum can be expressed as a function of the ratio of inductance -o-f thelower inductance coil andthe 'adjacent higher indue tance coil. `It is convenient to use the mid` y frequency of the secondary transmitting band i as a definition ofthe position tween the primary andsecondary transmit-A ting bands), depends also on theratio of the coil inductances.

(4J-The width of the `voice-frequency i band will be increased slightly.

Assuming that more inductance is taken from the large coiland added to the adjacent low 4inductance coil,`the"total remaining thesame, the four items mentioned above will be modified as follows:

(l) The additional band mentioned above willremain.

(2) Its width will be increased and its frequency lowered.. f

V(3)v The attenuation in the suppression band will be smaller. f n

`(li) The width 'of 2the yoice-frequency e band will'be increasedfurther.

If this Vprocess of. removing inductance from one coil and adding it to the other is continued until the inductance of both coils is the same, the compound loaded circuit will reduce, to a simple loaded circuit with coils having half the inductance and half the spacing of the original simple system. The characteristics of such a system would be as folows:

v,(1) The secondary transmitting band will remain untilthe inductance of the two coils becomes equal whereupon it willmerge with the primary transmitting band.

(2) Its width will continue to increase and its mid-frequencywill become lower until it mergeswith the primary band.

`(8) The attenuation of the suppression vband will continue to diminish and will finally become zero when the,secondary'transmission band Ibecomes confluent with lthe primary transmitting band; that is, the sup,- pressionband disappears.

(4)v The primary transmitting band be'- comes confluent with the secondary band and `this combined band has `a width Vtwice that.

of the original transmitting band. `[t `will be seenfrom` the above that the width yand 'mid-frequency c of the secondary transmitting channel can be controlled by Y merely varying the inductances of thetwo loading coils used in the compound system; However, it should be noted that if the ratio of inductances ofthe two coilsis varied, the primary critical frequency ofthe circuit will also vary. For a given loading coil spacing,

any two of the three factors, primary'band width, secondary band width or nominal impedance, may be fixed but not all three; for

' example, if it is desired to apply compound loading to a line so as to obtain a voice fre- `quency channelof a certain width and also to obtain a certain nominal line impedance,

it isnot possible to regulate thesecondary i plication of the invention to a system employing unequal inductances equally spaced along conductors of uniform capacity. The invention may also be .applied to the arrangement shown in Fig. 2 which represents a loading system having coils of equal inductance values and adjacent loading sections of unequal capacities. In this ligure the coils L3 have inductance values intermediate to those described in connection with Fig. 1. The sections-of the cable designated a have high capacity and those designated b have low capacity. y

rIhis'system behaves exactly the same as that shown in Fig. 1, ifit is assumed that instead of having loading sections with constant capacities and variable coil inductance ratios (as in l), the systemhas loading sections of constant inductance and variable capacity ratio. Comparing` this system with that of Figui onthe same basis:

(1,) Starting` with Zero capacity of the cable section o between the two coils in a compound loading section (equivalent to a simple loading system with two equal loading coils at the end of each section) and increasing the capacity in the same manner as the inductance in Figure l, the secondary transmitting band willbe introduced as soon as capacity is inserted between the two coils. The band will remain until the capacities between the coils are equal, in which case the compound loading becomes simple loading again.

(2) rlhe width of the secondary band will increase and its mid-frequency decrease until the band becomes confluent with the primary band.

(3) The 'attenuation of the suppression band will become successively smaller until the secondary band merges with the primary band. Y

(4) The width of the primary transmitting channel will increase until it merges with the secondary channel, its width then being twice the original value.

The arrangement shown in Fig. 3 represents a loading system in which the adjacent coils having unequal inductance and the adjacent loading sections have unequalcapacthe two loading coils are equal and when the spacings-are equal. Hence, starting with this as the eXtreme case, 'varyingeither the ratio of the inductances or capacities, or both, will cause the width of the band to decrease. As itsfwidth decreases, its mid-frequency will increase' and the attenuation of the suppression band-will'become larger and the width of the voice-frequency channel will y decrease until it reaches approximately'one-half its original value.

The method of obtaining a plurality of channels of transmission by means of compound loadingof a signaling circuit and the manner of cont-rolling the width of the band of Suppression separating the transmission bands 'will now 'be described.. rlhe description of the method will be limited to the system iof loading shown schematically in Fig. l inasmuch as this system is of the greatest practical importance.

The method consists ofaltern ating inductance loads of unequal values', tae rat-io m/ (1 -m) of the inductance units L1 and L2 tobe determined by the desired location ofthe .critical frequencies f1, f2 and f3, in accordance with the proportion The loweifband, the usual one, entends from O to f1, assuming` that m .5, while-a comparatively narrow additional transnn'ttingr band isintroduced fromfgto f3. The suppression band extends from flrto f2.

The line under consideration isa smooth (non-loaded) cable of propagation constant y perinile and of characteristic impedance 7c, with negligible distributed inductance and with loading inductance L1 and L2 of impedances Z1 and Z2, respectively, alternating at intervalsA of s miles as shown on the accompanying drawing. A complete section (including the loading` impedance .afl-a2) is then 2s mileslong. Such a system has, in general, three critical frequencies exclusive of zero, which divide the frequency range ,L s follows. `If L1 is the larger inductance, the lowest orI principalcrtical ,frequency f1 is that of a line with impedance loads Z1 at intervals of 2s miles. There is then an at,- tenuating band above this frequency and extending to the critical frequency f2, which is that of a line with loads of impedance Z2 at intervals of Qsiniles. The additional transmitting band extends from f2 to the third critical frequency f3, which `depends upon both loads. The region from f3' to oo is an attenuating band. y

The disposition of the transmitting and attenuating bands will be clear on reference to F ig. 4 of the accompanying drawing. Thus, when any inductanceL2 is introduced, a narrow transmitting band results. The position of this band on the frequencyy scale becomes lower `and its ,width somewhat widerV AMetroal 0] desir/a rEhe critical frequencies of the syste1n,.as stated above, are 1n the proportion,

rlhe percentage bandwidth W, delinedbelow, is then lt will be observed from Equations (l) and (2) that there are only two arbitrary constants (m and wo) so that only two of the critical frequencies may be arbitrarily fixed or the ratio of'any two determines the relative value of the third. In fact, the factor m determines the relative'disposition of the transmitting bands (as shown in Fig. 4) while, with it, the factor m., liXes them absolutely. Equations (l) and (Q) (or (2) and (3) do not completely determine the loading design, however, for only the product QLO Ca is liXed by them'. The separate values of total inductance L0 and total capacity 2C@ per ysection will be determined by practical considerations other than the location of the critical frequencies. Some examples of the application of the formulae follow.

Example 1 Given f1=4000 )2:8000 Then m= .8 andV giving a signaling channel 940 cycles wide.

' `bands in this case reaches VIt will be observed from the drawing that-.the

attenuation between thetwo transmitting about 1.4 units per sections of Qs'miles.l i* y y y Emample Z ..Required, V,asecondary channel of v10 rper cent band widthwith mid-frequency at20,000 cycles. r

Y .From

and from (1) since fg: 21,000 f1: 8,950 y y v Y vr'mample y y `Required, a `5 per cent bandl width with principal'riticalfrequency at` 41,000 cycles'.

` From (3) 1 and from (1) themidA-channel frequency is fai-.fai 1era/ it ,2 2W/bm 1421,600

The channelfis then 630 cycles wide and might be utilized Vas a telegraph or picturetransmission channel. An- S300-cycle channel with mid-frequency at 10,000 cycles (m=.852) is obtainable on a line with principal cut-off at 1,000 cycles and would be superior to the preceding for picture transmission. Y

Requireaf v l i f1=2,000

Thenfrom (f1), putting (f2-f2) /f1=1"v mhr2 2- 525 This is a oase of a very narrow attenuating band due to a comparatively slight difference between the loads. From formulae (2a) below,`however, it follows that the attenuation reaches .1 unit per section.` As the cut-off is sharp, this would be a quite distinct attenuating band. K y

i Derz'oatc'on of formula? Thefabove conclusions are based upon the derivation of the critical frequencies from the formula forthe propagation constant I of a 2s mile section of the system. The constant I is most easily derived by obtaining the difference equation of the Vvoltages as follows:

The voltage Vq inthe qth section is expressible as y VFM @Xp (fr- Q?) eXp. (QF) (4) Here M and N are arbitrary constants determined by the terminal conditions.

If the typical section is made up of two networks A and a (as the present line may be regarded) whose one and two point admittances are denotedv by All, A22, A12, A21 and an, U22, @12, @21, respectively, the following relations hold between the currents and voltages at the junctions kofthe networks. Iq and V2 i will denote the current and voltage at the junction of the qth and (g-l- 1)th sections, while q and cq denote the current and voltage respectively at the junction point of the two networks within the qth section. The currents L, and q at each of the two kinds of junction pointsvare expressed first in terms of the admittances of network A and then of network a. Thus,v K Y I [q:A11.Vq'-A21Vq+1:al12l7q(Z22-V11 and y Z'qfizAisVqAzz' Vq+1; a11

In the present instance the network a represents the smooth line of lengths miles and characteristics y, 7c terminated by an impedance Z1 and the network A-represents the smooth line of length s miles and characterist-ics y, le terminated by the impedance Z2. Hence the admittances are given the eX- pressions smooth cable of lengths s miles,

VFA @Xp (-ya) +B @Xp (ya) .fz- 11 @Xp we -B @Xp om the two terminal conditions RF1-Z310 determine the arbitrary constants A and B.

Substituting thesevalues in IX above gives 'l the required equationI IzziinO they value of @q obtainedl from Equation (6k) in Equation (5), thel latter becomes Where y (8) of the voltages, gives cosh Fte/9, (10) y y On substituting the valuesggiven byvEquareos, g= l w2m2L,Os l wm-3011,08) (15j e tion forthe admittances and simplify#Y ing, Equation' (l0) becomes Sinceaigagl and lzfbn'on substitut-vr tive. I Consequently, letting sent the incluctance and ly, of the smooth line per mile,

mima/ Ima/LT Where y ,q f

w=2rX frequency (13) A Furthermore, it is assumed that L is quite' small as compared to the loading inductance and so small, absolutely, that,

COS l sin Lux/L08 @VLC/S (14) Now in the transmitting range a is zero, so that from (12),

l cosh g cos Substituting thisand Equations (13) and (14) in (11) gives where Lo is the tomi loading mamme pei section q L=L1+L2), osgmgi,

ancl 5 COSh :1l/2 cosh y'ys vel-snh 787)'(cosh fyrs sinh ys) i (11) This is the required :formula -for the gation constant l".

propayRepresenting by a the attenuation and ,8 the phase changep'er-se'ction, gives cosh g= cosh g cos sinhzg sin To locate the critical frequencies, it is assumed, as usual, that the line is non-dissipa# L anoll C reprecapacity, respectivey A fzwhere Y where Equation-*(119) givesthe twovalues fof-f1 or.`

Stakes all valuesfrom 0 to unity, namely` fzgare simply interchangedL) `These are, ofr

where I-Iencethere aretwo ranges of frequency in which cos2 as determinediby Equation (15) q Offl, and,f2;f;f3. if m o.5, f1 and` course, the ranges of free transmission (a=0), while the range between f1 and 'f2 1s i an attenuati'ng band` It has been remarked above that f1 and f2 are the critical 'free quencies whichwould exist if one or the other of the loads were omitted and the remaining load retained only its allotted part of the inductance.

Y The limiting values m =`1 and m=0 reduce the system to the ordinary loadedv line with inductance units L0 at intervals'of'2s miles, fO'I'awhen/m:l,` Y- q n.

q 1 f2t=f3=w land. Y y v K i j *U1/2F00@ Y and when i E K lm=ov and q Y Q Y 1 h fV-'lrr 2L0O's In the special case m=0.5, making Z1=Z2h=z'w,5L0, the system becomes a line loaded at intervals of smiles with inductance 0.5 L0 and This value of f3 is the known critical frequency for such a system and since f1 and f2 are coincident they are eliminated as critical frequencies in this case, or, in other words,

To summarize, if the inductance L0 is all in the load Z1, i. e., m=1, the known critical frequency is given by f1, the others f2 and f3 being at infinity, or there is simply a transmitting bandvfrom zero to f1 and an attenuating band from f1` to infinity. Putting a VVsmall part of the load at thermid-point of the sec-y tion, i. e., making m' .5 and 1, moves f1 to a Aslightly higherY frequency and makes f2 and f3 finite, introducing a narrow transmitting band between the latter two. As Zl is continuously decreased and Z2 increased, f1 becomes continuously higher and Aboth f2 and f3 become continuously lower, the band besomewhat in breadth IFig. 4L illustrates the locations of the critical fre/quenciesufor different divisions of the total load of a compound system between the two adjacent loading points. In thelimiting` case above discussed, where 11F-:05D: e., L1=L2]4 the intermediate suppression band vanishes at a point on the frequency spectrum indicated in Fig. 1l, corresponding to; an abscissaJ value'of 1.414. Y

With regard to the magnitude of the attenuation, we return to Equation (11). In accordance Ywith the assumption that the line is non-dissipative, coshZF/2 is real as foundV above.A Then, when a is not zero, it must fol low that cos I-Ience cosh I/2 is pure imaginary, n is odd (by Equation (12)) and a 2 sinh/ e (1 C02/w12) (1 o2/w22) 24) 2 coshfl. when Wi== 0 Oni-the other hand, in the band above f3,

sincer 2 n 1s even and a= 2 coshlx/m (25) ien From (24) when m=0 or l, the slope (l a/d@ of the attenuation is da GO when or dw f f1 f2 and y I =0 when l-w2m (l-m) L0Us=0- The maximum value am of the attenuation in the lower band, occurring at the frequency fm, is then am=2 sinh"1 [i(1-2m)] (26) :0 when m=.5 A`

je.. f1 f2/2 1/m (i m) =j32/2 27) In the upper attenua-ting band, from Equation (25) when m=0 or 1,.

and

Vhen m=0 or l, not exist.

In summarization of what has beensaid f2: o@` and this band does before, as the coil'inductance values or the secondary transmitting band, the width of the suppression band diminishes. The design problem therefore involves compromise depending upon the intended purpose of the primary and secondary transmitting` channels.

While of course they may be used for various purposes the primary channel would ordinarily be preferred for speech transmission while the secondary channel could be used for carrier telegraph or for picture transmission.

It should be noted that a. plurality of secondary signaling channels may be obtained by increasing the complexity of the compound loading beyond that described hereinbefore. The unique attenuation characteristics of compound loading result from the superposition of two simple systems having a definite regularity in the staggering of the coils belonging to the two simple loading systems. By superposing with regular periodicitythree or more simple loading systems, the number of secondary transmitting channels may be materially increased.

Vhat is claimedA is:

l. A compound loaded transmission sys-V ingsystems, one superimposed on the. other,

the rsaid compound system vcomprising a line l of them the ad'acentl coils diiferine` in inductance values but the alternate coilsbeing of the same value, the relative separation of the coils Ybeing the same throughout the system'and the said valuesbeing so chosenthat the compound systeinwill beV characterized by two distinct, non-overlapping bandsV of transmission separated by a band of suppression. I v 2. vloaded transmission' system characterized by two bands of transmission separated by a band ofsuppression, the said system employing coils of unequal inductance vvalues Ll and L2 alternately connected and equally spaced throughout said system, the

Yvalues of inductance of the coilsbeing so,

chosen that Il n m L2 1.- m q where m is greater than .5 and less than l.

In a loaded transmission system the -combinationjvith a line of uniformly distributed capacity, of a plurality of loading coils, vof inductances L1 and L2 connected therein, the coils Ll being different in value from 'Iig and being alternately spaced with respect to L2, the distance between adjacent coils being the same throughout the transmission system, the inductance values being so chosen that Y L1 m L2n l m where m is greater than .5 and less than l.

et. A vcompound loaded transmission -system characterized by two bands of transmitted frequencies, separated by a band of suppressed frequencies, said system consisting of one simple loading system superposed on another simple loading system, the inductance of a coil of one system differing from thatk of a coil of the other system, with the loading coils of both systems occurring at regular spaced intervals, the said coils being so connected that the total line current flows through all of them.

5. A compound loaded transmission system characterized by two bands of transmitted frequencies, separated by a band of suppressed frequencies, said compound system consisting ofone simple loading system superposed on another simple loading system, the coils of both systems being equally spaced and'having inductance values of suoli magnitude and of proper ratio to produce transmission bands of desired width and position in the frequency spectrum, the said coils being so connected that vthe total line current Hows through all of them.

6. A compound loaded transmission system characterized by two bands of transmit ioo " the induotance ofthe loading coils ofthe `two systems being different and the relative spac-AV` ing of theooils ,being symmetrical Athrough- I v outthe Compound system,whe1ebythe Widths ofthe transmission-bends and their position i tedrequencies,'seperated by a bandi offsup-VV pressed frequencies,V said .compound systemy consisting of one simple loadingV system su'-V perposed 'on` another simple loading system,

` in the frequenoj7 s peotrum may beoontrolled,

"- the Said Coilsbeing so Conneetedthat the total line current ioWs through allzofthem. l

In testimony whereof, v'We have signed our names to Lthis specification this 17th dey of Ni A fJl

' Y' August, Y1926.

SALLIE 12;;MEA13.` i f NORMAN fyvFRENoH. 

